Basically, the electrocardiogram is the most important physiological parameter for patient monitoring. However, the pick-up of ECG signals may cause technical problems which are--although identified already a long time ago--not easy to overcome.
The most serious problem is caused by the fact that the patient may, for safety reasons, not be connected with the ground potential of the monitor. ECG monitors provide, in general, means for galvanic separation of the patient, such as transformers, optical couplers etc.
The external ECG is not measured against ground, but rather as a differential signal; i.e., one needs at least two leads, and measures the ECG waveform as a voltage difference between these leads. This is just because the patient ground is floating and thus not stable, such that any measurement against monitor ground would reveal useless results. It is understood that more than two leads may be provided as well, in order to obtain multiple waveforms.
The actual differential voltage of an external ECG signal is approximately in the range of 100 .mu.V to 10 mV. In contrast, the floating ground may cause common mode voltages (i.e., voltages appearing on both ECG leads) of up to (and even exceeding) 100 V. That is, the common voltages may be about 10.sup.3 to 10.sup.6 times larger in amplitude than the differential voltages.
It is thus not easy to keep the differential amplifier in its operating range, i.e., to keep its common mode rejection ratio (CMRR) high. A well-known measure therefor is to isolate the differential amplifier galvanically from ground, which results in a CMRR improvement of at most 70 dB. The limiting factor of this first measure is the insulation capacitance of the galvanic separation, which includes the insulation capacitance of the electrical circuitry, and the capacitance of the patient leads with respect to ground. However, the max. 70 dB CMRR are not sufficient to provide a differential signal of acceptable quality.
One has therefore to take other measures as well. One approach widely used in the prior art is the so-called "guard drive". In general, this means that the patient's ground potential is modified such that it approaches the ground potential of the "floating" portion of the pick-up circuitry. More specifically, a signal is derived from the differential signals, e.g. a sum signal, which controls an additional amplifier. The output of the additional amplifier is connected, via a further electrode, with the patient. This concept is also known as "right leg drive".
The contribution of the guard drive is a further improvement of the CMRR of about 20 dB. A limitation of this concept is the loop gain of the guard drive. However, its most serious disadvantage is that an additional electrode has to be provided; for example, 3 ECG electrodes have to be used to obtain a single lead (waveform).
The additional electrode is particularly disadvantageous in clinical practice, specifically in terms of handling. However, the additional time required to apply the third (or (n+1).sup.th) electrode, its cost and the confusion by an additional electrode cable is not the only aspect which limits the guard drive concept. Consider, for example, a neonate (newborn baby). A multiplicity of electrodes and sensors are usually applied to the neonate during monitoring in the intensive care unit (e.g., ECG, respiration, blood gases etc.). However, the space on the body of a newborn is limited, such that clinicians often have problems to apply all necessary electrodes.
Thus, there is a strong desire to remove the additional "guard drive" electrode. One has already developed ECG amplifiers without guard drive, so-called "two-lead" amplifiers. Prior art amplifiers of this type employ a differential amplifier with limited input resistance (as opposed to the virtually unlimited input resistance of the differential amplifier in the guard drive concept). The operating point of the differential amplifier is adjusted by means of resistors.
However, the latter concept imposes more stricter demands on the galvanic separation, as compared to the guard drive method. These stricter demands can only be met if the amplifier is integrated into the cable head of the patient cable (which makes the cable more bulky and uncomfortable to handle), and if a transformer of very low stray capacity (&lt;2 pF) is provided for galvanic separation. Both measures also contribute essentially to the costs of the amplifier, and last not least its differential characteristics is also impacted by the limited input resistance.
Thus, there is an ongoing need for a receiver circuit for ECG signals which operates without guard electrode, but still avoids the disadvantages of the known amplifiers.